Guide to Dispersants

What are Dispersants?

Oil dispersants are a blend of solvents and surfactants. The solvent is designed to reduce the viscosity of the surfactant so that it can be sprayed more easily and it assists with the penetration of the surfactant into the oil where it breaks down the surface tension between the oil and water which maintains the oil on the surface in the form of a slick.  


Source: ITOPF

How Do Dispersants Work? 

Dispersants are designed to breakdown the surface tension between the oil and surrounding water and the result of successful chemical dispersion is the creation of very small neutrally buoyant oil droplets which are amenable to natural bio-degradation.  This process of biodegradation is much enhanced when the oil is dispersed into small droplets as the dispersed oil surface area is much increased when compared with the same oil volume when in the form of a surface slick.                                                        

Does Dispersed Oil Sink to the Bottom?

No - Dispersed oil is neutrally buoyant and rarely sinks below 10m in the water column as illustrated in the graph above.  This is generally why the use of dispersant is restricted by water depth although there may be circumstances where dispersant is authorised for use in shallow waters or even on inter-tidal zones.  Local regulations on the use of dispersants must be observed as the consideration of local conditions will take account of:

  • Size of the spill and type of oil
  • Sensitive habitats
  • Mariculture activites
  • Wind and Tidal effects
  • Other available oil spill response techniques etc

When used properly and in a controlled manner, dispersants will “help nature to do its work” by significantly increasing the rate of biodegradation and preventing the oil from reaching the shoreline where it can have a major environmental impact.

Why Use Dispersants? 

Almost all parties involved in oil spill response will prefer to use oil spill response techniques which will simply remove the spilt oil from the environment.  So mechanical containment and recovery of the oil must be the preferred option.  However, in many occasions, this is difficult and often impossible when sea conditions are too rough and when the size of the spill overwhelms mechanical techniques.  At Ayles Fernie we see dispersants as complementary to mechnical oil recovery methods and dispersants represent another “tool” in the oil spill responders toolkit;  a tool which should always be available.

How Much Dispersant is Required?

This is a more difficult question to answer. 

Window of Opportuity. Oil is amenable to dispersion when the dispersant is applied as soon as possible after the spill.  The longer the period between the spill and the application of dispersant, the more difficult it will be to disperse the oil successfully.  We call the period between spilling the oil and the point at which the oil cannot be successfully dispersed, the “Window of Opportunity”.  The reason that spilt oil becomes more difficult to disperse with time is that the oil will evapourate some components, there will be a tendancy for the oil to emulsify and the oil temperature will reduce to that of the sea all of which will tend to increase the oil viscosity.

There are other factors which affect the effectiveness of dispersant such as:

  • Oil Type
  • Sea Temperature
  • Condition of the Oil

DOR. In general it is sensible to assume that spilt oil will disperse if the dispersant is applied at a ratio of about 1:20 that is 1 part of dispersant to 20 parts of oil.  We call this the DOR ie the Dispersant to Oil Ratio.  As the oil emulsifies and weathers the required DOR for dispersion may be more like 1:10 or even 1:5.  But how do you determine how to apply dispersant at a DOR of 1:20 for example?

Consider the diagram below which shows an area of oil called a hectare which is 100m x 100m.  Now assume a slick thickness of 0.1mm which is a typical thickness and the volume of oil in this conveniently sized slick is 1000 litres.



Application Rate. As shown in the diagram,  if you decide to use a DOR of 1:20 then, providing the slick is 0.1mm thick, then an Application Rate of 50 Litres/Hectare will achieve the desired result.  We have used a new term here – “Application Rate” and it is this term that we use in our system manuals as it is easy to understand as a basis for operating spraying equipment.

Not an Exact Science.  Much useful work has been carried out in the scientific community in relation to the use of dispersants.  Although this work is valuable, the use of dispersants is not an exact science because of the number of variables which affect the successful process of dispersion.  We mention a few above such as:

  • Oil Type
  • Emulsification
  • Weathering
  • Temperature
  • Viscosity
  • Slick Thickness etc, etc.

Slick Thickness is very difficult to determine so assumptions need to be made which is why we advocate a “Spray and Observe” approach unless more sophisticated methods are available. Our guidance for operators is to try a low Application Rate – say 50 Litres/Hectare, observe the results making adjustments to the Application Rate as necessary.

What Type of Equipment Should I Use?

The best type of equipment will depend on the size and location of the spill.  Marine systems require an appropriate vessel platform and large spills offshore may justify a response with aircraft systems.

With the modern emphasis on Risk Analysis and Contingency Planning, these techniques should identify the most likely size and location of the spill, together with the type of oil and this information will assist with selecting the most appropriate equipment.

Marine Systems

Portable Pump Units. Small diesel powered portable pump units provide the flexibility for oil compaies or oil spill responders who need the ability to respond to spills in many and even world wide locations.  This flexibility demands a self-contained and independent pumping system which does not rely on other power sources.  The Ayles Fernie BOATSPRAY product range includes diesel pump units which can be operated on small inshore craft and larger offshore vessels.View BOATSPRAY Systems

Installed Pump Units.  For vessels which are involved in the oil industry or are exposed to operations with a high risk of oil spills such as Offshore Support Vessels (OSVs of all types), Tugs, Crew Boats and other Workboats, it makes sense to equip these vessels with an installed and more permanaent system which has the advantage of being set up and ready to operate at short notice.  These vessels are very likely to be on the scene of a spill and are best placed to respond quickly.  Our view is that all OSVs should be equipped with an installed dispersant spray system in the same way that a fire fighting capability is standard on this class of vessel.  The Ayles Fernie solution for installed  dispersant pump units is the CLEARSPRAY product range and these pump units are available for small through to large vessels. View CLEARSPRAY Systems

Spraying Equipment.  The conventional method for applying dispersant is from Spray Arms with multiple nozzle positions to provide even coverage.  To cover large areas more quickly, and given that spraying speeds from boats and ships is likely to be at about 5 – 10 knots, the only way achieve a high Area Treatment Rate (in km2/hr) is by extending the spray patern or Swath Width.  For spray arms this means longer spray arms with the added weight and complexity that results.

Ayles Fernie can supply standard spray arms from 4m up to 10m and special designs can be undertaken.  Spray Arms remain the most controlled method for applying dispersant but they have significant disadvantages ie weight, size, cost, special installation fittings and lack of flexibility.

AFEDO Nozzles.  In response to strong market demands, Ayles Fernie develped the AFEDO Nozzle in 2005 and this device has become very popular as an alternative to Spray Arms.  Whilst the spray pattern and droplet size is not as controlled as with spray arms, the qualties of the AFEDO Nozzle outweigh the advantages fo spray arms where operational flexibility is important.  In particular they are designed for “Vessel of Opportunity” applications ie where the operator needs to be able to move the system from vessel to vessel.  When operated with portable BOATSPRAY pump units, the operator will gain maximum operational benefit from the AFEDO Nozzle solution


Special Designs.  Ayles Fernie can design more specialised spraying solutions and retractable 10m spray arms are now being developed for high freeboard, enclosed foredeck vessels which are becoming popular for modern OSVs operating offshore in more demanding sea conditions

Equipment Features. Dispersant spray systems must be designed to apply dispersants at a variety of Application Rates as explained earlier.  The Application Rate is related to vessel speed, swath width and dispersant flow rate.  The vessel speed can be set, the swath width is fixed so the operator must be able to vary dispersant flow rates.  Also, the spray arm nozzle set up or AFEDO Nozzles are designed to operate at a fixed flow rate in order to generate the correct droplet size.  So it is important that the pumping system is equiped with control valves and Flow Meters which will allow the operator to properly control the dispersant flow and hence Application Rates.

All Ayles Fernie dispersant pump units are available with Flow Meters or Flow Rate Indicators (FRIs) for portable BOATSPRAY models.

Neat or Dilute Spraying.  Ayles Fernie systems can be supplied with the capability to spray concentrated (Neat) or Diluted dispersant.  Although diluted dispersant is less efficient for dispersing oil, it has some advantages for applying dispersant to fresh amenable oil.  AFI recommends the use of diluted dispersant for situations where low application rates are required.  The main advantage of dilute dispersant application is that the dispersant flow rate can be varied whilst retaining the same outlet flow rate.  This means that the same nozzles can be used for a wide range of dispersant (Application Rates) flows.

System Options.  Ayles Fernie systems are offered with a wide range of options so that the equipment can be optimised to suit customer requirements.  There are options which we regard as almost essential (Flow Meters for example) but we provide this purchasing flexibility to give our customers the ultimate choice in configuring their systems.

Aircraft Systems

For larger spills and in conditions which are unsuitable for ship dispersant application aircraft dispersant systems are another option. The much higher speed of aircraft means that the Area Treatment Rate is some 15 times greater than that of ships so for spills affecting a large area, the use of aerial spraying has a clear advantage.

Aircraft dispersant systems are designed and certificated for specific aircraft types and models and the current Ayles Fernie aircraft systems include:


For the Lockheed L382G “Hercules” aircraft



for the Airbus C295 aircraft

What Dispersant Should I Use?

We would recommend that any dispersant which has been tested and approved by a recognised authority ie UK DEFRA, EPA, Cedre for example, will be suitable and Ayles Fernie systems are compatible with all approved dispersants.

We will be happy to advise further on dispersants and Ayles Fernie can quote and supply their recommended dispersant SUPERDISPERSANT-25  if required


Some excellent documents exist which will provide much more detail on the subject of dispersants.  This short treatment of the subject is meant as a basic introduction only.

Ayles Fernie will be happy to advise further and if you have any questions do contact us.

Recommended Reading is as follows:

  • IMO/UNEP Guidelines on Oil Spill Dispersant Application  - Including Environmental Considerations
  • ITOPF Technical Information Paper No. 4 – Use of Dispersants to Treat Oil Spills
  • IPIECA Report Series Vol Five – Dispersants and Their Role in Spill Response

Ayles Fernie International

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